docs: decorate file name

Author: kenjis

README.md

@@ -33,7 +33,7 @@ You might also be interested in the [API documentation](https://codeigniter4.git
 
 ## Important Change with index.php
 
-index.php is no longer in the root of the project! It has been moved inside the *public* folder,
+`index.php` is no longer in the root of the project! It has been moved inside the *public* folder,
 for better security and separation of components.
 
 This means that you should configure your web server to "point" to your project's *public* folder, and